This invention relates to anti-theft devices for motor vehicles and more particularly to such devices for incorporation in the hydraulic braking systems of motor vehicles.
Theft of motor vehicles, particularly domestic motor cars, is a major social problem, and numerous antitheft and alarm devices are currently available to the motorist for protecting his vehicle. However most known devices suffer from one or more disadvantages.
Electrically-operated devices can fairly readily be de-activated by appropriate disconnection from the electrical supply. In this respect, access to the associated electrical supply can be achieved from beneath the car and without raising undue suspicion by a potential thief. Furthermore, audible and/or visual alarm devices may deter a thief from entering a vehicle, but do not in any way de-activate the vehicle itself which can be removed once entry is achieved.
Steering locks, although efficient, can be picked or `popped`, particularly with the sophisticated tools currently available.
There have also been proposals, in particular with the wide adoption of hydraulic braking circuits, for immobilising a motor vehicle by locking its brakes on. By closing the return path for the hydraulic fluid from the brake cylinders using a security key, while permitting the driver then to apply hydraulic pressure to the brake cylinders, the brakes can be locked on until the security key is released. It is fundamental, however, that the safety of a vehicle braking system must not be compromised. The problem exists with this form of system of providing an arrangement which not only effects the locking operataion reliably over a long surface life but which does not carry any risk of impairing the operation of the brake system when the vehicle is being driven.
As examples of earlier proposals for security locking menas for motor vehicle brake circuits, reference can be made to GB 449095, 732042, 1104432 and 2024351A, EU 0064124A and 0239987A, US 3579285 and WO85/04845.
In GB 732042, GB 2024351, EU 0064124A and EU 0239987A, the hydraulic fluid return path is closed by spring-loaded ball valves, but these cannot be relied upon to provide completely fluid-tight sealing over a prolonged period; moreover, if there is any corrosion with the passage of time they bring the risk that the valve ball will stick in its seating, so increasing the braking effort needed, or in an extreme case, making it impossible to open the fluid path in either direction. An analogous case is provided by GB 449095, GB 1104432 and US 3579285 where the return flow is blocked by spring-loaded poppet valves. In the example of WO85/04815, an axially slidable spindle valve is employed to control the flows between a multiplicity of ports in a valve housing and the possibility of a hydraulic short circuit developing through loss of sealing efficiency would bring the danger of failure of hydraulic brake circuit while the vehicle is being driven.